1. Field of the Invention
The present invention generally relates to optical position recognition, and more particularly to methods, systems, apparatuses, and computer program products for employing optical position recognition techniques to correlate frame data acquired during multiple measurements (i.e., captured representations, such as, e.g. images or scans) of an object for use in obtaining a three-dimensional representation of the object.
2. Related Art
In conventional three-dimensional measurement systems, such as those having a small field of view used to obtain images of relatively larger objects, for example dental structures such as actual or prosthetic teeth or dental molds or castings, the measuring field or the measuring volume of the optical measurement system is smaller than a volume of the object to be measured. Accordingly, it is necessary to perform multiple measurements of different portions of the object to acquire substantially complete representations for the object. The object is moved relative to the optical measurement system between measurements. The data acquired from each measurement must be correlated, i.e., mapped onto a common coordinate system, to obtain a composite three-dimensional representation of the entire object.
Conventional three-dimensional measurement systems may employ mechanical registration techniques to correlate data acquired during multiple measurements. FIG. 10A depicts an exemplary system 1000 that uses conventional mechanical registration techniques to correlate three-dimensional data acquired during multiple measurements. The system 1000 includes measuring optics 1002 and a slide 1004. A support member 1006 positions the measuring optics 1002 at a fixed orientation relative to the slide 1004, such that there is no relative movement between the measuring optics 1002 and the slide 1004. A mechanical grid 1008 is provided on an upper surface of the slide 1004. An object 1010 is secured to an object holder 1012. The object holder 1012 is positioned in predetermined locations on the mechanical grid 1008. A measurement is performed and a frame of three-dimensional data is acquired at each location. A composite three-dimensional representation of the entire object is created by combining the frame data according to well-known frame registration techniques. A disadvantage of the system 1000 is that the object holder 1012 can be placed only in predetermined locations that are accommodated by the mechanical grid 1008, which may not be optimal locations for acquiring three-dimensional data.
Conventional three-dimensional measurement systems also may employ optical registration techniques to correlate frame data from multiple measurements. Positions are determined by points of reference located on an object holder. A Cercon Eye Scanner from DeguDent GmbH employs optical registration techniques, for example.
FIG. 10B depicts an exemplary system 1050 that uses conventional optical registration techniques to correlate three-dimensional data acquired during multiple measurements. The system 1050 includes measuring optics 1052 and a slide 1054. A support member 1056 positions the measuring optics 1052 at a fixed orientation relative to the slide 1054, such that there is no relative movement between the measuring optics 1052 and the slide 1054. An object 1058 is secured to an object holder 1060. The object holder 1060 includes a reference position marker adjuster 1061 that positions a reference position marker 1062 above the object 1058. The object holder 1060 is then moved over the slide 1054 in discrete steps. A measurement is performed and a frame of three-dimensional data is acquired during each step. Each measurement must include the reference position markers 1062. Optical registration techniques are used to identify the reference position marker 1062 and generate corresponding positioning information for each frame of three-dimensional data. A composite three-dimensional representation of the entire object is created by combining the frame data according to well-known frame registration techniques.
The measuring optics 1052 typically include a camera (not illustrated) that is employed to observe the reference position marker 1062 on the object holder 1060. A disadvantage of the system 1050 is that the camera must be able to view the reference position marker 1062 during each measurement. The reference position marker 1062 must not be covered by the object 1058 or otherwise obscured from the camera while measurements are taken.
Calibration patterns with circular structures have been used to calibrate measuring optics of photogrammetry systems, such as the systems shown in FIGS. 10A and 10B. These calibration patterns typically consist of unfilled circles having a constant radius, which are placed on a rectangular grid. Conventionally, a calibration pattern fits entirely within a field of view of the measuring optics to be calibrated. Because the entire calibration pattern is used to determine metrics of the measuring optics, it is not necessary to determine coordinates of a position on the calibration pattern based on a partial view of the calibration pattern.
The mentioned calibration patterns may be generated by offset printing on ceramic substrates. However, because offset printing may be reproducible on a scale of several 10 microns, the resulting calibration patterns are not sufficient for a calibration process. Each such calibration pattern can be calibrated by an independent measuring process, and a look-up table generated for every calibration pattern. Each look-up table contains exact positions of centers of every circle. Calibration of measuring optics using such a calibration pattern requires the full calibration pattern to be within the field of view of the measuring optics, if the look-up table is to be used. Accordingly, features of calibration patterns are not encoded for such applications.
Other methods may be used to generate such calibration patterns. For example, processes that are similar to semiconductor lithography processes for producing semiconductor devices, may be used to generate a calibration pattern. With these processes, a mask may be imaged onto a photo-resist resin disposed on top of a thin chrome layer located on a quartz substrate. The substrate is then illuminated and, in a subsequent etching step, regions that are not illuminated (i.e., masked) are removed. In other lithographic processes, a photo emulsion may be applied to a mylar foil, for example. Such lithographic processes results in small deviations of only a few microns over a total area of one square inch. Accordingly, it is possible to generate highly accurate calibration patterns using such lithographic processes.
As mentioned above, when measuring optics are calibrated using conventional calibration processes, the measuring optics acquire an image of a complete calibration pattern. Thus, conventional calibration processes do not employ position recognition techniques to determine a position on the calibration pattern using acquired image data corresponding to a portion of the calibration pattern. However, position recognition techniques that use partial information of a search pattern are known. For example, position recognition techniques may be used in conjunction with a two-dimensional bar code encoded according to a Data Matrix standard (ISO/EC 16022—International Symbology Specification). A two-dimensional bar code is generated by encoding information with a high degree of redundancy. A partial image of the two-dimensional bar code may be acquired, and error correcting algorithms may be used to determine a position corresponding to the acquired image. Two-dimensional bar codes, however, are not optimized to allow initially unknown information to be extracted. That is, two-dimensional bar codes are not optimized for an absolute position to be determined when only partial bar code information is known.
The present invention overcomes the above limitations associated with measuring a three-dimensional object using conventional frame registration techniques.